Petrography

Petrographic descriptions of Soufriere eruptive rocks have been given by Roobol & Smith, (1975), Carey & Sigurdsson, (1978), Shepherd et al., (1979), Graham & Thirlwall, (1981), Devine & Sigurdsson, (1983), Dostal et al., (1983) and Bardintzeff, (1984, , 1992). Most of these studies concentrated on the products of the activity since 1902.

Detailed sample and locality descriptions of rocks used in this study are given in Appendix A. Soufriere is formed almost entirely of basalts and basaltic andesites [classification scheme of LeBas et al., (1986)]; andesites are found only as components of mixed magma rocks of the Yellow Tuff Formation. Both basalts and basaltic andesites have been erupted throughout the volcano's history, although basalts were volumetrically at their most abundant in the earlier stages (Pre-Somma and Yellow Tuff Formations). Roobol & Smith, (1975) recorded a progressive change from early erupted basaltic andesite to late basalt in the 1902-1903 activity, perhaps pointing to the existence of zoned magma chambers at least spasmodically beneath Soufriere.

Table A1 shows the modal proportions (volume percent, recalculated vesicle- and groundmass-free) of phenocrysts in representative samples from each of the main geological formations of Soufriere. The basalts range from microphyric, fine-grained rocks with abundant (up to 30%) microphenocrysts of ol + spinel ± cpx, to more coarsely porphyritic rocks also containing phyric plagioclase. Basaltic andesites are generally phenocryst rich (35-60%), containing the assemblage cpx + Ti-mag + plag + opx ± ol. The order of appearance of phases was ol + Cr-sp ± Ti-mag, followed by cpx, plag and then opx.

Although it forms a core to an augite crystal in STV 303, pigeonite occurs mainly as thin rims around orthopyroxene, clinopyroxene and olivine crystals in the Pre-Somma Lavas (STV 315, 318, 323). Titanomagnetite microphenocrysts are present in some basalts but are always subordinate to Cr-spinel. Ilmenite is present only as a groundmass phase. Amphibole has not been recorded as a phenocryst at Soufriere, though Shepherd et al., (1979) and Graham & Thirlwall, (1981) found corroded amphiboles in products of the 1979 eruption which were assumed to be xenocrystic. We have found rounded crystals of amphibole in STV 354 (from the 1979 eruption) and STV 363 (scoria of unknown age from the Pyroclastic Formation). The amphiboles are 2 mm long, greenish yellow in colour, and contact plagioclase crystals. The aggregates probably represent parts of cumulate blocks. Only one occurrence of phyric apatite has been found, as a microphenocryst included in a plagioclase phenocryst in andesite STV 376(L).

There is a marked tendency for the phenocryst phases to form clusters, up to 2 mm across. These vary from monomineralic, clinopyroxenitic clots through olivine-clinopyroxene-rich clusters to titanomagnetite-rich gabbros. STV 358 contains a 4 mm * 3 mm olivine aggregate where the crystals are strained and partially recrystallized.

Melt (glass) inclusions occur in all the phenocryst phases (Graham & Thirlwall, 1981; Devine & Sigurdsson, 1983; Bardintzeff, 1992). Those in olivine tend to be large (50-100 µm) and spheroidal, and generally have large contraction bubbles. There is a tendency for inclusions in olivines in the most magnesian basalts to be at least partly devitrified. Inclusions in orthopyroxene are more abundant and larger ( <= 50 µm) than those in clinopyroxene ( <= 30 µm), whereas in plagioclase large ( <= 100 µm) subrectangular to spheroidal inclusions form trains in crystal cores.

Macroscopic evidence (e.g. banded and mingled pumices) for mixing between basaltic andesite and dacite at Soufriere has been documented for the 1902 (Carey & Sigurdsson, 1978) and 1979 (Shepherd et al., 1979; Graham & Thirlwall, 1981; Devine & Sigurdsson, 1983; Bardintzeff, 1992) eruptions. Light-coloured bands of dacite in scoria blocks in the 1979 ejecta contain feldspar, orthopyroxene, quartz and magnetite crystals, and some contain partially fused granitic xenoliths (Graham & Thirlwall, 1981; Devine & Sigurdsson, 1983). They are evidence of at least local crustal fusion beneath Soufriere. We have also found clear evidence of mixing between basalt and andesite and between basaltic andesite and andesite in western outcrops of the Yellow Tuff Formation and in scoria from the Pyroclastic Formation.

Cumulate-textured blocks are found at most volcanoes in the Lesser Antilles, but are particularly abundant and well documented at Soufriere, St Vincent. Mineralogy and textures are highly variable, but hastingsitic amphibole, calcic plagioclase, olivine, titanomagnetite and high-Ca pyroxene are common cumulate phases (Lewis, 1973a, 1973b; Arculus & Wills, 1980; Dostal et al., 1983).

Metavolcanic and calc-silicate sedimentary xenoliths are also common at Soufriere (Devine & Sigurdsson, 1980; Carron & Le Guen de Kerneizon, 1991). We have analysed five metamorphic xenoliths to help constrain the nature of any potential crustal assimilant; the protoliths were tuffs (STV 336, 340), basalt (STV 353), and andesite (STV 337, 339). STV 337 is cordierite bearing.

Phenocryst compositions

In this section, we refer to various mineral-melt partitioning data, using whole-rock compositions as proxies for melt compositions. We appreciate that this creates problems in strongly porphyritic rocks, in that later stages of phenocryst growth may have been from melts widely removed in composition from the bulk rock. We have tried to take account of this effect by using only phenocryst core compositions, which we assume to have crystallized close to the liquidus. Mineral compositional data for Soufriere samples are available from R.M. on request.

Olivine phenocrysts in the most magnesian basalts (e.g. STV 301) take three forms: (1) euhedral to subhedral prisms up to 1 mm in size, with core compositions ranging from Fo₈₉ to Fo₈₇ and substantial zoning (up to 19% Fo); (2) smaller (<0·5 mm) rounded crystals, with slightly more Fe-rich cores (Fo_86-83) and zoning <14% Fo; (3) rare, ragged, variably resorbed, probably xenocrystic, grains, with cores of Fo₈₀ and only weak, normal zoning (~3% Fo). No reverse zoning has been found; this suggests that crystallization occurred in a system where there was no, or minimal, input of fresh magma. In the basaltic andesites, olivines are small (<1 mm) and commonly embayed through resorption; mantling textures indicate an olivine-orthopyroxene reaction relationship. Core compositions range from Fo₈₅ to Fo₅₅, with rare values of Fo₉₀. NiO contents reach 0·38% and are positively correlated with mg-number.

The most magnesian olivines (mg-number ~89) are found as phenocrysts in the most magnesian basalts, e.g. STV 301. This rock has an mg-number of 73 [using an Fe₂O₃/FeO weight ratio of 0·22, derived from the Kilinc et al., (1983) expression for Fe speciation, and an f(O₂) of FMQ (fayalite-magnetite-quartz) + 1·7 (see below)]. Olivine of composition Fo₈₉ could have crystallized from melt with mg-number 73 if K_D = 0·33. This value is the same as that determined by Wagner et al., (1995) for olivines grown in 1 kbar, water-saturated experiments on a high-Al₂O₃ basalt from the Medicine Lake volcano, California, and similar to the average value of 0·34 determined for the 1 kbar water-saturated experiments of Sisson & Grove, (1993b). Ulmer, (1989) has demonstrated experimentally that Mg-Fe²⁺ partitioning between olivine and liquid in a calc-alkaline picrobasalt is pressure dependent, and reported a range of K_D from 0·315 at 1 bar to 0·365 at 25 kbar. The value for 15 kbar, close to the possible equilibration pressure of 17 kbar for STV 301 (see Figs 8 and 9, below) was 0·341. We conclude, therefore, that STV 301 represents the most primitive melt erupted by Soufriere. Even more primitive rocks have been recorded from other arcs; Eggins, (1993) has collated compositions of olivine phenocrysts from several arc systems, some as high as Fo₉₄.

On plots of Fe²⁺/Mg ratios of olivine phenocrysts against whole rocks (Fig. 3a), core compositions, in particular, plot on both sides of any reference K_D line, that is, they apparently have compositions that are either too evolved (above the line), or too primitive (below) for the whole-rock composition. There are several possible explanations for the scatter:

 (1) The P-T-X dependence of K_D (Ulmer, 1989) means that K_D should constantly change to lower values as magma evolves, and not remain constant as is commonly assumed.

 (2) Relatively primitive core compositions in basaltic andesites, e.g. Fo₈₆ in SVE 113 (SiO₂ 54·8 wt %) may represent phenocrysts which did not re-equilibrate from higher-temperature stages of magma evolution, and/or the products of mixing with a more primitive basaltic magma. We note, however, that reverse zoning is uncommon in Soufriere phenocrysts.

 (3) The compositions of the rims of many olivines are more Fe rich than could be predicted from equilibrium relationships (Fig. 3a). This again may reflect magma mixing, this time with a more evolved melt, or differential amounts of re-equilibration of olivine microphenocrysts by solid-state diffusion at magmatic temperatures.

Figure 3. (a-c) Average Fe²⁺/Mg in cores and rims of olivine, clinopyroxene and orthopyroxene phenocrysts, respectively, plotted against Fe²⁺/Mg in host rocks. Continuous lines are for equilibrium K_D values. (d) Plot of average Ca/Na (cations) in cores and rims of plagioclase phenocrysts and in host rocks. Continuous lines represent approximate exchange K_D, which show a progressive increase with melt H₂O content from 1·0 for anhydrous melts through ~1·7 for melts with 2 wt % H₂O and 3-4 for melts with 4 wt % H₂O to 5·5 for melts with 6 wt % H₂O (Sisson & Grove, 1993a, fig. 1).

In all rock types, a spinel phase forms small (0·2 mm) euhedral to subhedral inclusions in the cores of olivine and, less frequently, pyroxene phenocrysts, and also occurs more rarely as partially resorbed, discrete microphenocrysts, the size increasing from <0·5 mm in basalts to 0·8 mm in basaltic andesites. There is a continuum of compositions from Cr-spinel to titanomagnetite, although the majority of analyses tend to be bimodal (Fig. 4); in Cr-spinel, cr-number ranges from 35 to 85 (the majority >50), mg-number from 12 to 54 and Fe³⁺/(Fe³⁺ + Al + Cr) from 10 to 80. TiO₂ varies from 0 to 10 wt %. In titanomagnetites, the same ranges are 0-35 (most <10), 5-26 and 81-98, respectively, and TiO₂ varies from 5 to 30 wt % (Fig. 4). Compositional zoning has been measured in only a handful of larger microphenocrysts and shows Mg/Fe, Cr/Al and Fe²⁺/Fe³⁺ ratios decreasing, and TiO₂ abundances increasing, towards crystal rims. Differentiation within the high-temperature spinels led to ferrite and ulvöspinel enrichment with little change in cr-number (Fig. 4a). This is typical of crystallization under relatively oxidizing conditions (Ballhaus et al., 1991). There is a crude, positive correlation between Fe²⁺/Mg ratios in spinels and whole rocks, suggesting an approach to equilibrium crystallization.

Figure 4. (a) Variation of cr-number [100 Cr/(Cr + Al)] with mg-number [100 Mg/(Mg + Fe²⁺)] in Cr-spinels and titanomagnetites present as phenocrysts and inclusions in Soufriere rocks; (b) Cr-Al-Fe³⁺ variation in Cr-spinel and titanomagnetite phenocrysts and inclusions in Soufriere rocks, compared with fields for spinels in island arc basalts (IAB) presented by Eggins, (1993, fig. 6).

The Soufriere spinels are similar to spinel compositions recorded from island arcs elsewhere; on a Cr-Al-Fe³⁺ plot (Fig. 4b), for example, they largely fall within the fields of arc basalts constructed by Eggins, (1993). The compositional range, and particularly the continuum between Cr-spinels and titanomagnetites, closely matches spinels from picrites of Ambae volcano in the Vanuatu arc (Eggins, 1993).

The majority of clinopyroxene phenocrysts are augite, although diopsidic cores are found in olivines and augites in more magnesian rocks, e.g. STV 301 and STV 334. They are typically 0·5-2 mm in size, subhedral, with weak zoning, partially resorbed margins and inclusions of Fe-Ti-Cr oxides. There is a strong tendency in some rocks for pyroxene and olivine crystals to form clusters 2 mm in diameter.

Maximum Cr concentrations ( <= 0·9 wt %) are found in clinopyroxene cores in some more magnesian basalts (STV 301, 334); values in more evolved rocks are typically <0·1 wt %. The Soufriere clinopyroxenes are notably Al rich, with maximum a.f.u. values >0·25 (Al₂O₃ > 6 wt %) in some basalts. Variation in Al with mg-number in the clinopyroxenes (Fig. 5a) shows a diffuse peak at mg-number 80. This is similar to the situation in ankaramites from western Epi (Barsdell & Berry, 1990) and in picrites from Ambae (Eggins, 1993), both in the Vanuatu arc. In all three cases, the peak coincides with the commencement of plagioclase crystallization. The clinopyroxenes in Soufriere basalts reach relatively high Al^VI values ( <= 0·13 a.f.u.). Although such high values might indicate high-pressure crystallization (Kennedy et al., 1990), the fact that the distributions of Al^IV and Al^VI as a function of mg-number are closely similar to that of [Sigma]Al suggests that the Al^IV/Al^VI ratio is composition dependent. Ti behaviour in Soufriere clinopyroxenes (Fig. 5b) fairly closely mimics that of Al, as it does in the Ambae suite (Eggins, 1993), though the abundances in Soufriere basalts tend to be higher.

Figure 5. Variations in (a) Al and (b) Ti cations per six oxygens as a function of mg-number in clinopyroxene phenocrysts.

Compositional zoning is most commonly normal, with rimwards enrichment in Fe (<5% Fs) and decrease in Cr, Al and sometimes Ti. Reverse zoning is restricted to some basaltic andesites and is typically at the limit of analytical resolution (Fs <= 1%).

Clinopyroxene and whole-rock compositions correlate fairly well, suggesting that quasi-equilibrium conditions prevailed during clinopyroxene crystallization (Fig. 3b). Relationships are consistent with an equilibrium K_D around 0·4 in the basalts and 0·35 in more evolved rocks. These values are higher than those (~0·20-0·25) found experimentally in 1 atm, anhydrous experiments on mid-ocean ridge basalt (MORB) compositions and silica-saturated and -undersaturated arc lavas (Grove & Bryan, 1983; Grove & Baker, 1984; Kennedy et al., 1990) and 1 kbar water-saturated experiments by Sisson & Grove, (1993b), but comparable with that of 0·38 determined in an Aleutian high-magnesia basalt at 12 kbar, 1315°C under anhy